There is a conventional variable displacement hydraulic actuator unit, including a hydraulic actuator such as a hydraulic motor or a hydraulic pump, and a volume adjusting mechanism that changes a volume of the hydraulic actuator, wherein an operating force that actuates the volume adjusting mechanism is generated by an electrically-operated motor.
This sort of variable displacement hydraulic actuator unit is configured such that the electrically-operated motor is actuated based on a difference between a current volume of the hydraulic actuator (hereinafter, referred to as a “current volume”) and a target volume thereof (hereinafter, referred to as a “target volume”), thereby matching the current volume to the target volume and maintaining the matched state.
Incidentally, the volume adjusting mechanism is always biased by a hydraulic pressure of an operating fluid toward a neutral side. Furthermore, the variable displacement hydraulic actuator unit may include a neutral biasing mechanism that biases the volume adjusting mechanism to the neutral side. In this manner, the biasing force to the neutral side is applied as an external force to the volume adjusting mechanism. Furthermore, in addition to the biasing force, an external force to the neutral side may be applied by some sort of factor to the volume adjusting mechanism or the hydraulic actuator.
When such an external force is applied to the volume adjusting mechanism, a problem occurs in which the volume of the hydraulic actuator becomes unintentionally offset from the target volume. Accordingly, in order to prevent this problem from occurring and to maintain a state in which the volume of the hydraulic actuator matches the target volume, a force that can counteract the external force (hereinafter, referred to as a “counteracting force”) is necessary.
Examples of conceivable means for obtaining the counteracting force include means for continuously supplying a required electrical power to the electrically-operated motor even after the volume of the hydraulic actuator has matched the target volume, and means for stopping the electrical power supply to the electrically-operated motor and obtaining a force of inertia (internal resistance) of the electrically-operated motor as the counteracting force.
However, if the former means is used, a large amount of electrical power is consumed, and, moreover, heat generated by the electrically-operated motor may cause operational errors, malfunctions, and the like.
On the other hand, if the latter means is used, the above-described problem still occurs when an external force exceeding the force of inertia is applied to the volume adjusting mechanism.
The present invention has been made in view of these conventional techniques, and it is an object thereof to provide an electrically-operated hydraulic actuator unit and a hydraulic four-wheel drive work vehicle in which, while allowing a volume adjusting mechanism to be actuated by an electrically-operated motor, it is possible to prevent a volume of a hydraulic actuator from being unintentionally changed by an external force applied to the volume adjusting mechanism or the hydraulic actuator.